European Research Council (ERC), the Quantum Integrated
Photonics (QUANTIP) project, A Toolbox for Photon Orbital
Angular Momentum Technology (PHORBITECH) project, the
Quantum InterfacES, SENsors, the Communication based on
Entanglement (Q-ESSENCE) integrating project, Nokia, the
Centre for Nanoscience and Quantum Information (NSQI), the
Templeton Foundation, and the European Union Union
Device-Independent Quantum Information Processing (DIQIP)
project. J.L.O. and S.P. acknowledge a Royal Society Wolfson
Merit Award. A.P. holds a Royal Academy of Engineering
Research Fellowship.

Supplementary Materialswww.sciencemag.org/cgi/content/full/338/6107/634/DC1Materials and MethodsFig. S128 June 2012; accepted 18 September 201210.1126/science.1226719Entanglement-EnabledDelayed-Choice ExperimentFlorian Kaiser,1 Thomas Coudreau,2 Pérola Milman,2,3 Daniel B. Ostrowsky,1 Sébastien Tanzilli1*Wave-particle complementarity is one of the most intriguing features of quantum physics. Toemphasize this measurement apparatus–dependent nature, experiments have been performedin which the output beam splitter of a Mach-Zehnder interferometer is inserted or removed aftera photon has already entered the device. A recent extension suggested using a quantum beamsplitter at the interferometer’s output; we achieve this using pairs of polarization-entangledphotons. One photon is tested in the interferometer and is detected, whereas the other allowsus to determine whether wave, particle, or intermediate behaviors have been observed. Furthermore,this experiment allows us to continuously morph the tested photon’s behavior from wavelike toparticle-like, which illustrates the inadequacy of a naive wave or particle description of light.

Although the predictions of quantum me- chanics have been verified with marked precision, subtle questions arise when
attempting to describe quantum phenomena in
classical terms (1, 2). For example, a single quantum object can behave as a wave or as a particle.
This concept is illustrated by Bohr’s complementarity principle (3) which states that, depending
on the measurement apparatus, either wave or
particle behavior is observed (4, 5). This is demonstrated by sending single photons into a Mach-Zehnder interferometer (MZI) followed by two
detectors (Fig. 1A) (6). If the MZI is closed [that
is, if the paths of the interferometer are recombined at the output beam splitter (BS2)], the probabilities for a photon to exit at detectors Da and
Db depend on the phase difference q between
the two arms. The which-path information remains
unknown, and wavelike intensity interference patterns are observed (Fig. 1B). On the other hand,
if the MZI is open (i.e., if BS2 is removed), each

photon’s path can be known, and consequently, no
interference occurs. Particle behavior is said to be
observed, and the detection probabilities at Da
and Db are equal to ½, independent of the value
of q (Fig. 1C). In other words, these two different
configurations—BS2 present or absent—give different experimental results. Recently, Jacques et al.
have shown that, even when performing Wheeler’s
original gedanken experiment (7) in which the
configuration for BS2 is chosen only after the
photon has passed the entrance beam splitter BS1,
Bohr’s complementarity principle is still obeyed
(8). Intermediate cases, in which BS2 is only partially present, have been considered in theory and
led to a more general description of Bohr’s complementarity principle expressed by an inequality
limiting the simultaneously available amount of
interference (signature of wavelike behavior) and
which-path information (particle-like behavior)
(9, 10). This inequality has also been confirmed
experimentally in delayed-choice configurations
(11, 12).

We take Wheeler’s experiment one step fur-ther by replacing the output beam splitter by aquantum beam splitter (QBS), as theoretically pro-posed of late (13, 14). In our experiment (Fig. 2),we exploit polarization entanglement as a re-source for two reasons. First, doing so permitsimplementing the QBS. Second, it allows usto use one of the entangled photons as a testphoton sent to the interferometer and the otherone as a corroborative photon. Here, as opposedto previous experiments (8, 11), the state of theinterferometer remains unknown, as does thewave or particle behavior of the test photon, untilwe detect the corroborative photon. By continuous-ly modifying the type of measurement performedon the corroborative photon, we can morph thetest photon from wave to particle behavior, evenafter the test photon was detected. To excludeinterpretations based on either mixed states, as-sociated with preexisting state information (15),or potential communication between the two pho-tons, the presence of entanglement is verified viathe violation of the Bell inequalities with a space-like separation (16–18).